In recent years polymeric materials have become available that possess heat resistance and strength properties previously found only in metals. Additionally, the polymers are much lighter than metals, an important advantage where weight is a factor as in modern, high speed aerospace applications. While it is possible to tailor a polymer system for a given application, the processing problem has been a restrictive factor in limiting the use of high temperature resistant polymers.
In processing a polymer into a composite structure, the polymer must flow in order to impregnate the reinforcing substrate and mold it to the desired form. The lower the softening point (Tg) or the melting point (Tm) of a polymer the easier it is to cause the polymer to flow. While it is desirable that a polymer have a low softening point, a composite fabricated from such a polymer loses its strength at temperatures approaching its softening point. In order that such a composite may be suitable for use at temperatures higher than the polymer's softening point, a procedure is required for subsequently raising the softening point of the polymer higher than the desired maximum use temperature.
The conventional method of raising polymer softening points is to cure the polymer by joining new chemical bonds or crosslinks between polymer chains. In the curing method often utilized, a trifunctional monomer is used in the polymer synthesis to provide crosslinking sites along the polymer backbone. This method often leads to branching and gelation during synthesis or storage of prepreg solutions. Other methods for accomplishing crosslinking include radiation, addition of a free radical source, incorporation of a pendant group which can react thermally or chemically, and thermal scission of C-H bonds in the polymer backbone.
There are three principal disadvantages to the crosslinking method of cure. One disadvantage results from the evolution of volatiles from any type of cure in which a condensation reaction is used. Because volatiles are evolved, voids are formed by entrapped gases, effectively weakening the composite structure. A second disadvantage derives from the brittleness which is inherent in a three-dimensional network. The third disadvantage lies in the fact that the softening point is raised only as high as the cure temperature because of "freezing in" of the reactive sites when the polymer softening point reaches the cure temperature. In other words, the polymer begins to soften as the use temperature approaches the cure temperature.
In U.S. Pat. No. 3,876,814, F. L. Hedberg and F. E. Arnold disclose quinoxaline polymers having pendant phenylethynyl groups. Because of the presence of these groups, the polymers can be cured to polymers which can be used at temperatures above this cure temperatures. Furthermore, in the heating operations during which the polymers are cured, there is no evolution of volatile by-products.
It is a principal object of this invention to provide polymeric materials which are precursors for synthesizing polyaromatic keto-ether-sulfones having pendant phenylethynyl groups.
Another object of the invention is to provide a process for preparing the precursors.
A further object of the invention is to provide polymers having pendant phenylethynyl groups which cure without the evolution of volatiles and which in the cured state exhibit softening points above their cure temperatures.
Still another object of the invention is to provide a process for preparing polyaromatic ether-keto-sulfones having pendant phenylethynyl groups.
Other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying disclosure and the drawing in which FIGS. 1, 2 and 3 show Vicat softening curves for uncured and cured polymers of the invention.